Many identification systems are known which involve an interrogator transmitting a microwave or other radio frequency signal and one or more transponders, which reradiate the transmitted frequency, either unmodified or modified to some degree. For example, the interrogator may be a radar located in an aircraft and the transponder may be located on the ground or attached to a vehicle. In a military environment, such devices serve as useful sensors in battlefield “friendly fire” control. Commonly, such devices include programmable encryption circuitry operative to regulate the signal characteristics of the transponder signal in accordance with an encryption code.
Various enhancements to transponder systems have been proposed which enhance the effectiveness of those systems. For example, contemporary transponders utilize frequency conversion techniques whereby the transponder radiates a reply signal at a frequency different from the radar interrogation signal. In such a manner, the transponder signal is more easily segregated from the interrogation signal and reflected ground clutter. Other conventional enhancements include the use of coherent transponders that allow the reply signal to phase track the interrogation signal.
Pulse compression techniques are commonly used in radar systems to allow the use of long pulses to achieve high-radiated energy, while obtaining the range resolution of short pulses. For devices to communicate well via radio frequency (RF) signals, the carrier signals in the transmitted and received signals must be well-matched. In this context, well-matched signals are signals that have the same frequency and phase code. With respect to pulse compression of modulated signals, this matching is critical for adequate signal processing gain and data extraction at the receiver. Phase locked loops have been used in transponder designs to match the frequencies of received and transmitted signals. While theses designs are useful in some applications, they may not work well enough for devices with very low effective radiated power levels and at extreme operating ranges.
The Joint Surveillance Target Attack Radar System (Joint STARS) is a long-range, air-to-ground surveillance system designed to locate, classify, and track ground targets in all weather conditions. While flying in friendly airspace, Joint STARS can look deep behind hostile borders to detect and track ground movements in both forward and rear areas. Joint STARS includes an airborne platform with a multi-mode radar system and mobile Ground Station Modules (GSMs). The airborne platform carries a phased-array radar and is capable of providing targeting and battle management data to all Joint STARS operators, both in the aircraft and in the ground station modules.
There is a need for a data link that enables matching of transmitted and received signals in systems that include transponders that operate with low effective radiated power, such as the Joint STARS system.